A device may have input means that can be used to receive transmitted signals from the surrounding environment. For example, a device may have audio input means such as a microphone that can be used to receive audio signals from the surrounding environment. For example, a microphone of a user device may receive a primary audio signal (such as speech from a user) as well as other audio signals. The other audio signals may be interfering (or “undesired”) audio signals received at the microphone of the device, and may be received from an interfering source or may be ambient background noise or microphone self-noise. The interfering audio signals may disturb the primary audio signals received at the device. The device may use the received audio signals for many different purposes. For example, where the received audio signals are speech signals received from a user, the speech signals may be processed by the device for use in a communication event, e.g. by transmitting the speech signals over a network to another device which may be associated with another user of the communication event. Alternatively, or additionally, the received audio signals could be used for other purposes, as is known in the art.
In other examples, a device may have receiving means for receiving other types of transmitted signals, such as radar signals, sonar signals, antenna signals, radio waves, microwaves and general broadband signals or narrowband signals. The same situations can occur for these other types of transmitted signals whereby a primary signal is received as well as interfering signals at the receiving means. The description below is provided mainly in relation to the receipt of audio signals at a device, but the same principles will apply for the receipt of other types of transmitted signals at a device, such as general broadband signals, general narrowband signals, radar signals, sonar signals, antenna signals, radio waves and microwaves as described above.
In order to improve the quality of the received audio signals, (e.g. the speech signals received from a user for use in a call), it is desirable to suppress interfering audio signals (e.g. background noise and interfering audio signals received from interfering audio sources) that are received at the microphone of the user device.
The use of stereo microphones and other microphone arrays in which a plurality of microphones operate as a single audio input means is becoming more common. The use of a plurality of microphones at a device enables the use of extracted spatial information from the received audio signals in addition to information that can be extracted from an audio signal received by a single microphone. When using such devices one approach for suppressing interfering audio signals is to apply a beamformer to the audio signals received by the plurality of microphones. Beamforming is a process of focusing the audio signals received by a microphone array by applying signal processing to enhance particular audio signals received at the microphone array from one or more desired locations (i.e. directions and distances) compared to the rest of the audio signals received at the microphone array. For simplicity we will describe the case with only a single desired direction herein, but the same method will apply when there are more directions of interest. The angle (and/or the distance) from which the desired audio signal is received at the microphone array, so-called Direction of Arrival (“DOA”) information, can be determined or set prior to the beamforming process. It can be advantageous to set the desired direction of arrival to be fixed since the estimation of the direction of arrival may be complex. However, in alternative situations it can be advantageous to adapt the desired direction of arrival to changing conditions, and so it may be advantageous to perform the estimation of the desired direction of arrival in real-time as the beamformer is used. Adaptive beamformers apply a number of “beamformer coefficients” to the received audio signals. These beamformer coefficients can be adapted to take into account the DOA information to process the audio signals received by the plurality of microphones to form a “beam” whereby a high gain is applied to the desired audio signals received by the microphones from a desired location (i.e. a desired direction and distance) and a low gain is applied in the directions to any other (e.g. interfering or undesired) signal sources. The beamformer may be “adaptive” in the sense that the suppression of interfering sources can be adapted, but the selection of the desired source/look direction may not necessarily be adaptable.
As described above, an aim of microphone beamforming is to combine the microphone signals of a microphone array in such a way that undesired signals are suppressed in relation to desired signals. In adaptive beamforming, the manner in which the microphone signals are combined in the beamformer is based on the signals that are received at the microphone array, and thereby the interference suppressing power of the beamformer can be focused to suppress the actual undesired sources that are in the input signals.
As well as having a plurality of microphones for receiving audio signals, a device may also have audio output means (e.g. comprising a loudspeaker) for outputting audio signals. Such a device is useful, for example where audio signals are to be outputted to, and received from, a user of the device, for example during a communication event. For example, the device may be a user device such as a telephone, computer or television and may include equipment necessary to allow the user to engage in teleconferencing.
Where a device includes both audio output means (e.g. including a loudspeaker) and audio input means (e.g. microphones) then there is often a problem when an echo is present in the received audio signals, wherein the echo results from audio signals being output from the loudspeaker and received at the microphones. The audio signals being output from the loudspeaker include echo and also other sounds played by the loudspeaker, such as music or audio, e.g., from a video clip. The device may include an Acoustic Echo Canceller (AEC) which operates to cancel the echo in the audio signals received by the microphones.
Although the AEC is used to cancel loudspeaker echoes from the signals received at the microphones, a beamformer (as described above) may simplify the task for the echo canceller by suppressing the level of the echo in the echo canceller input. The benefit of that would be increased echo canceller transparency. For example, when echo is present in audio signals received at a device which implements a beamformer as described above, the echo can be treated as interference in the received audio signals and the beamformer coefficients can be adapted such that the beamformer applies a low gain to the audio signals arriving from the direction (and/or distance) of the echo signals.